Unable to render high definition image
You are looking for a higher image resolution. The default resolution is 1920x1080, which means the image is 1920 pixels (little color blocks) sideways, and 1080 pixels up and down. Doubling each to get 3840x2160 will mean that what used to be one pixel will now be four (from 1x1 to 2x2).
Samples is how much data goes into one pixel. In Cycles, a sample is one ray trace from a pixel out backwards into the world, which will bounce uniformly on a shiny object but arbitrarily on a rough object. Too few samples makes rough things like stone look fuzzy, but things like polished metal generally look alright. More samples means more realistic renders, more computation time, and more desire to use GPU compute possibly .